1. Field of the Invention
This invention generally relates to electrochemical cells and, more particularly, to a battery that uses a low temperature molten salt (LTMS) as part of the cathode.
2. Description of the Related Art
A battery is an electrochemical device in which ions and electrons commute between an anode and a cathode to realize electrochemical reactions. The voltage and capacity of the battery are determined by its respective electrode materials. In general, metals anode materials promote a high voltage in the battery, while their low molecular weights provide a large capacity. For example, lithium has the most negative potential of −3.04 V vs. H2/H+ and the highest capacity of 3860 mAh/g. High voltage and large capacity lead to an overall high energy for the battery. In addition, sodium, potassium, magnesium, nickel, zinc, calcium, aluminum, etc. are good candidates as anode materials in metal-ion batteries.
The state-of-the-art cathode materials focus on the metal-ion host compounds. Metal-ions can be extracted from the interstitial spaces of the electrode materials in the charge process, and inserted into the materials during the discharge process. However, it is worth noting that the charge/discharge process severely distorts the lattice of the materials, which essentially destroys their structures following several cycles. Moreover, these cathode materials can only provide less than one tenth the capacity of the metal anode materials. Therefore, new cathode materials need to be developed in order to both match the higher capacities of the anode materials and enable longer cycle lives for the metal-ion batteries.
In 1996, Abraham and Jiang reported a polymer electrolyte-based rechargeable lithium/oxygen battery in which oxygen was used as the cathode material [K. M. Abraham, Z. Jiang, A polymer electrolyte-based rechargeable lithium/oxygen battery, Journal of the Electrochemical Society, 143 (1996) 1-5]. Oxygen (in air) continuously flows into the battery and provides a very high specific energy of 5200 Wh/kg. Nevertheless, the oxygen cathode has several disadvantages. Firstly, electro-catalysts were used in the batteries to reduce the kinetic barrier for the oxygen reactions. Secondly, the sluggish electrochemical reactions of oxygen produce a large overpotential in the lithium/air battery. Thirdly, the cathode of the lithium/air battery has to remain open to allow air access. Similarly, an oxygen cathode has also used in zinc-air batteries [Philip N. Ross, Jr., Zinc electrode and rechargeable zinc-air battery, U.S. Pat. No. 4,842,963].
In 2011, Lu and Goodenough revealed an aqueous cathode for a lithium-ion battery [Yuhao Lu, John B. Goodenough, Youngsik Kim, “Aqueous cathode for next-generation alkali-ion batteries”, Journal of the American Chemical Society, 133 (2011) 5756-5759]. They used aqueous solutions of water-soluble redox couples (for example, Fe(CN)63−/Fe(CN)64−) as the cathode. The lithium/aqueous cathode battery operated at ca. 3.4 volts in an ambient environment. The battery demonstrated a small overpotential, a high coulombic efficiency, and a long cycle life. However, water is an inert material in the electrochemical system, which reduces the specific capacity of the cathode. Although the design of a lithium/flow-through cathode battery can increase the capacity and energy, its volume must necessarily be large.
During the same period, Carter and Chiang disclosed a patent that described the use of a flowable semi-solid composition (slurry) as the electrode materials in batteries [William C. Carter, Yet-Ming Chiang, High energy density redox flow device, US 2011/0189520]. The electrode included at 5% vol % of the total volume, active materials to store energy. The inventors demonstrated an example where the cathode slurry of 25 vol % lithium cobalt oxide, 0.8 vol % carbon black, 73.2 vol % electrolyte was tested in a lithium/slurry cathode battery. The composition of slurry produced a suitable viscosity for the device design. Only 0.36 Li-ion can reversibly insert/deinsert into/from a LiCoO2 molecule between 2 V and 4.5 V because there is a small contact area between the solid particles and the current collector. Moreover, it's inevitable that the solid particles have to suffer considerable strain when the ions insert into their lattice, which restricts the cycle life of the electrode material. According to the data, it can be determined that the concentration of the active material is 7.2 M while the specific capacity of the slurry is 69.22 Ah/L. With respect to the molten salt of Fe(NO3)3.9H2O, for example, its capacity is 98.4 Ah/L (90% of the theoretical capacity) which is larger than that of the slurry electrode materials. In addition, the slurry electrode has a high viscosity that causes a significant loss of parasitic energy. Therefore, the slurry battery exhibits a low energy efficiency.
Although the appearance of molten sodium batteries does not adequately address the issues and challenges associated with cathode materials for metal-ion batteries, they are mentioned in order to adequately distinguish molten sodium batteries from the low-temperature molten salt cathode described in detail below. The rechargeable molten sodium batteries are mainly either sodium-sulfur batteries or sodium-nickel chloride (ZEBRA) batteries. A sodium-sulfur battery consists of molten sodium at the anode and molten sulfur at the cathode. In the discharge process, sodium ions transport from the anode to cathode and form Na2S4. The voltage of the battery is ca. 2 volts. Similar to the Na—S battery, a ZEBRA battery uses the molten sodium as anode, but the molten NiCl2 acts as the cathode. The electrochemical reaction in the battery is 2NaCl+Ni⇄NiCl2+2Na. Its operating voltage is around 2.4 volts. One significant problem is that the molten salt batteries of Na—S and Na—NiCl2 have to operate at a high temperature (270° C.) in order to maintain the salts in the melt state.
It would be advantageous if a battery could be fabricated with a cathode that included a salt that remains molten at low temperatures.